Doepfer A-188-1CV BBD Delay 2048 Stages

Description

Module A-188-1 is a so-called Bucket Brigade Device module (abbr. BBD). BBDs have been used to delay audio signals before digital delays dethroned the BBD based effect units. But BBDs have some very unique advantages (or disadvantages dependent on the point of view) over the digital counterpart which result from the special properties of the BBDs. BBD circuits can be treated as a chain of Sample&Hold units (S&H) which pass on their voltages to the next S&H in the chain at each clock pulse. A more detailed explanation – including the different types of BBDs – can be found in following chapter.

In any case the sounds generated by module A-188-1 are very special. Typical applications are: Flanger, Chorus, Analog Delay or Karplus/Strong synthesis. But as the A-188-1 has a lot of very unique features (voltage controlled clock rate / delay time with extreme range, polarity switches for the CV inputs, feedback and BBD out/mix, clock and CV output of the high speed VCO, BBD clock input, feedback insert, feedback up to self-oscillation) a lot of unusual applications can be realized with the module (e.g. delay controlled by ADSR, envelope, random or sequencer with positive or negative effect). The A-188-1 also has no built-in anti-alisaing filter in order not to limit the possibilities of the module. For this the CV out is intended.

A more detailed description of the module can be found in the user’s manual A1881_man.pdf.

Turning the manual delay knob clockwise or an increasing the external CV with polarity switch in positive position increases the clock and consequently Decreases the delay time (similar to a VCF or VCO, especially because of Karplus-Strong synthesis applications)

Clock output (e.g. to control another BBD, SC-Filter or delay module)

CV output (e.g. to control a VCF that follows the BBD clock), this voltage is composed of the manual delay control, CV1 and CV2

BBD/audio section:

Standard versions of the module: 1024 or 2048 stage BBD (choice at order, a mark at the front panel identifies the type of BBD, similar to A-138a/b)

Special versions of the module: 128, 256, 512 or 4096 stage BBD (additional charge, available only while stocks of these special BBD chips last)

The delay ranges for the 128, 256, 512 and 4096 stage versions can be found in the user’s manual A1881_man.pdf.

The max. delay time (???) is not specified as it depends upon the desired quality of the delayed audio signal. The signal becomes more and more poor as the Clock frequency is reduced (please refer to the corresponding note below and listen to the audio examples for details)

External clock input (used to control the module e.g. from an external HSVCO or from another BBD module). The socket is normalled to the output of the internal clock oscillator.

Two audio inputs (connected as a miniature multiple) with attenuator

BBD output (raw delayed output without original signal, e.g. for external feedback control via other A-100 modules, e.g VCA or VCF), the BBD output is affected by the feedback polarity switch to obtain the polarity function even for external feedback loops

Mix control (relation between original signal and positive or negative BBD signal)

Mixed audio output

The main difference between the two types of BBDs is the delay time range. The maximum clock frequency mentioned in the data sheet of the 2048 stage BBD is 100kHz. This leads to a minimum delay time of about 10ms. But we found that the device is able to operate even a bit beyond this spec (up to ~ 150kHz) causing a minimum delay time of about 7ms. The maximum clock frequency mentioned in the data sheet of the 1024 stage BBD is 200kHz. This leads to a minimum delay time of about 2,5ms. Overclocking is not possible for the 1024 stage BBD.

The maximum (useful) delay time is hard to define. The output becomes more and more crunchy as the clock frequency goes down because the number of samples per second taken from the original signal decreases as the clock frequency is reduced. But this is only one aspect. Indeed several effects occur simultaneously:

Kind of bit crunching because of the increasing time between samples taken from the audio signal

Aliasing effect as the sample rate becomes less than twice the max. audio frequency

BBD losses: the BBDs are not specified for clock frequencies less than 10 kHz (this value is valid for both 1024 and 2048 stage BBDs). The BBDs can be treated like a chain of sample&hold units with very small capacitors (the capacitors are integrated on the BBD chip). Clock frequencies below 10kHz cause voltage losses as the holding times of the S&Hs are no longer sufficient to pass on the voltages to the succeeding stage without errors/losses.